Robust current loop controller for servo system

ABSTRACT

A robust current loop controller for a servo system is provided. The robust current loop controller includes a model reference controller and a current controller. The model reference controller generates a velocity command signal from a current command reference signal of the servo system and generates a current command signal through comparing the velocity command signal with a feedback velocity command signal of the servo system. The current controller generates a control signal from the current command signal, the current command reference signal and a current feedback signal in order to drive the servo system.

FIELD OF THE INVENTION

[0001] The present invention relates to a current loop controller. Inparticular, the present invention relates to a robust current loopcontroller for a servo system.

BACKGROUND OF THE INVENTION

[0002] Please refer to FIG. 1, which is a block diagram of a typicalservo system. A typical servo system 100 includes the motor system 101,the current controller 102, and the velocity controller 103. Thetransfer function of the motor system 101 includes a coil transferfunction, G_(a)(S)=1/(Ls+R), and a mechanism transfer function,G_(J)(s)=1/(Ls+R), where L denotes the winding inductance, R denotes thewinding resistance, J denotes the equivalent rotor inertia of the motorsystem, and B denotes the equivalent damping coefficient of the motorsystem. The transfer function of the current controller 102 is G_(c)(s).The transfer function of the velocity controller 103 is G_(s)(s). ω(s)denotes the rotation rate of the motor, i_(f)(s) denotes the feedbackcurrent, i_(a)(s) denotes the current command of the motor and v_(r)(s)denotes the velocity command of the motor.

[0003] In the typical servo control system 100, the current and thevelocity of the motor system 101 is controlled by the current controller102 and the velocity controller 103, respectively. Generally, since thecoil parameters L, R of the motor system 101 are only influenced by thetemperature, the change of the coil parameters would be small if thetemperature is not too high. Also, the current loop of the prior servosystem is approximately constant under 500 Hz. However, the mechanismparameters J, B are changed with different loads.

[0004] Please refer to FIG. 2, which is a Bode Plot diagram for thevelocity open loop of the typical servo control system. As shown in FIG.2, while the rotor inertia J increases, the Bode gain of the open loop(dB) decreases owing to considering that the velocity of the velocitycontroller is fixed. This enlarges the steady-state error and theactive-state error of the whole servo system. Therefore, it is necessaryto have an additional velocity controller with higher gain in order tomeet the requirement of the servo control properties. In order toovercome this problem of the typical driver, the estimator is used forobtaining an inertia value, and then an appropriate velocity controlleris provided. However, the solution is only suitable for which theinertia changes slow. For which the inertia changes fast, the solutionwill cause bad actions of the servo system.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a robustcurrent loop controller for a servo system. A model reference controlleris provided for controlling a rotor inertia which is changed accordingto the load of the servo system to be approximate to the reference valueof the rotor inertia of the motor.. The robust property is stillprovided by the current loop.

[0006] It is an further object of the present invention to provide arobust current loop controller for a servo system. A model referencecontroller is provided for inputting the difference in value beween themodel output and the angular valocity of the rotor of the motorimmediately to the current controller in order to resist the externalinterference of the servo system. The robust property against theexternal interference is provided by the current loop.

[0007] It is an further object of the present invention to provide arobust current loop controller for a servo system. When there isresonance in the servo system, a model reference controller is providedfor inhibiting the resonance automatically.

[0008] In accordance with one aspect of the present invention, thecurrent loop controller for the servo system of the present inventionincludes: a model reference controller generating a velocity commandsignal from a current command reference signal of the servo system andgenerating a current command signal through comparing the velocitycommand signal with a feedback velocity command signal of the servosystem; and a current controller generating a control signal from thecurrent command signal, the current command reference signal and acurrent feedback signal in order to drive the servo system.

[0009] Preferably, the servo system is an AC servo system.

[0010] Preferably, the servo system is a permanent magnet servo system.

[0011] Preferably, a transfer function of the model reference controlleris K_(t)/(J_(m)s+B_(m)), wherein J_(m) is a reference of a rotor inertiaof a motor, B_(m) is a damping coefficient of a motor and K_(t) is aratio.

[0012] Preferably, a rotor inertia control which is changed according toa load of the servo system is controlled by the model referencecontroller to be approximate to the reference of the rotor inertia ofthe motor.

[0013] Preferably, J_(m) and B_(m) of the transfer function,K_(t)/(J_(m)s+B_(m)), are according to a default of a specification.

[0014] Preferably, the specification is a steady-state error of theservo system.

[0015] Preferably, the current command signal is generated by the modelreference controller according to a difference between the velocitycommand signal and the feedback velocity command signal.

[0016] Preferably, the control signal is a voltage-controlled signal.

[0017] Preferably, the control signal is a current-controlled signal.

[0018] In accordance with another aspect of the present invention, themethod for controlling a current loop in a servo system includes stepsof: generating a velocity command signal from a current commandreference signal of the servo system through a first operation;generating a current command signal through comparing the velocitycommand signal with a feedback velocity command signal of the servosystem; and generating a control signal from the current command signal,the current command reference signal and a current feedback signalthrough a second operation in order to drive the servo system.

[0019] Preferably, the servo system is an AC servo system.

[0020] Preferably, the servo system is a permanent magnet servo system.

[0021] Preferably, a transfer function of the model reference controlleris K_(t)/(J_(m)s+B_(m)), wherein J_(m) is a reference of a rotor inertiaof a motor, B_(m) is a damping coefficient of a motor and K_(t) is aratio.

[0022] Preferably, a rotor inertia control which is changed according toa load of the servo system is controlled by the model referencecontroller to be approximate to the reference of the rotor inertia ofthe motor.

[0023] Preferably, the current command signal is generated by the modelreference controller according to a difference between the velocitycommand signal and the feedback velocity command signal.

[0024] Preferably, the control signal is a voltage-controlled signal.

[0025] Preferably, the control signal is a current-controlled signal.

[0026] The foregoing and other features and advantages of the presentinvention will be more clearly understood through the followingdescriptions with reference to the drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWING

[0027]FIG. 1 is a block diagram of the typical servo system according tothe prior art;

[0028]FIG. 2 is a Bode Plot diagram for the velocity open loop of thetypical servo control system according to the prior art;

[0029]FIG. 3 is a block diagram illustrating the robust current loopcontroller for the servo system in accordance with a preferableembodiment of the present invention; and

[0030]FIG. 4 is a Bode Plot diagram for the velocity open loop of therobust current loop controller for the servo system in accordance with apreferable embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention will now be described more specificallywith reference to the following embodiments. It is to be noted that thefollowing descriptions of preferred embodiments of this invention arepresented herein for purpose of illustration and description only; it isnot intended to be exhaustive or to be limited to the precise formdisclosed

[0032] Please refer to FIG. 3, which is a block diagram illustrating therobust current loop controller for the servo system in accordance with apreferable embodiment of the present invention. As shown in FIG. 3, thecurrent loop controller 202 includes the model reference controller 204and the current controller 205. The velocity command signal ω_(a)(s) isgenerated by the model reference controller 204 from the current commandreference signal i_(a)(s) of the servo system 200. The current commandsignal i_(r)(s) is generated through operating the difference in valuebetween the velocity command signal ω_(a)(s) and the feedback velocitycommand signal ω(s) of the servo system 200. The control signal isgenerated by the current controller 205 from the current command signali_(r)(s), the current command reference signal i_(a)(s) and the currentfeedback signal i_(f)(s) in order to drive the servo system 201. Theservo system 201 can be represented by the coil transfer function 206and the mechanism transfer function 207.

[0033] The servo system may be an AC servo system or a permanent magnetservo system. The transfer function of the model reference controller isK_(t)/(J_(m)s+B_(m)), where J_(m) is the reference of the rotor inertiaof the motor, B_(m) is the damping coefficient of the motor and K_(t) isthe ratio. The rotor inertia control which is changed according to aload of the servo system is controlled by the model reference controllerto be approximate to the reference of the rotor inertia of the motor.

[0034] Furthermore, the reference of the rotor inertia of the motor andthe damping coefficient of the motor of the transfer function,K_(t)/(J_(m)s+B_(m)), may be set according to the steady-state error,the active-state error or the response rate, etc. of the servo system.

[0035] Moreover, the current command reference signal i_(a)(S) isgenerated by the velocity controller according to the difference betweenthe velocity command signal ω_(a)(s) and the velocity command referencesignal v_(r)(s). The velocity command signal ω_(a)(S) is the outputrotation rate ω(s) of the servo system which the servo system 200feedbacks to.

[0036] Please refer to FIG. 4, which is a Bode Plot diagram for thevelocity open loop of the robust current loop controller for the servosystem in accordance with a preferable embodiment of the presentinvention. Owing to the properties of the model reference theory, thelow-frequency gain of the Bode Plot for the velocity open loop does notchange with the change of the rotor inertia.

[0037] Therefore, the robust current loop controller in accordance withthe present invention is provided for controlling the rotor inertiawhich is changed according to the load of the servo system to beapproximate to the reference value of the rotor inertia of the motor,and the robust property is still provided by the current loop.Furthermore, it is possible to input the difference in value beween themodel output and the angular valocity of the rotor of the motorimmediately to the current controller in order to resist the externalinterference of the servo system, and the robust property against theexternal interference is provided by the current loop. Also, when thereis resonance in the servo system, the model reference controller inaccordance with the present invention is provided for inhibiting theresonance automatically. Hence, the present invention not only has anovelty and a progressive nature, but also has an industry utility.

[0038] While the invention has been described in terms of what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention need not be limited to thedisclosed embodiment. On the contrary, it is intended to cover variousmodifications and similar arrangements included within the spirit andscope of the appended claims which are to be accorded with the broadestinterpretation so as to encompass all such modifications and similarstructures. Therefore, the above description and illustration should notbe taken as limiting the scope of the present invention which is definedby the appended claims.

What is claim is:
 1. A current loop controller for a servo systemcomprising: a model reference controller generating a velocity commandsignal from a current command reference signal of said servo system andgenerating a current command signal through comparing said velocitycommand signal with a feedback velocity command signal of said servosystem; and a current controller generating a control signal from saidcurrent command signal, said current command reference signal and acurrent feedback signal in order to drive said servo system.
 2. Acurrent loop controller according to claim 1 wherein said servo systemis an AC servo system.
 3. A current loop controller according to claim 1wherein said servo system is a permanent magnet servo system.
 4. Acurrent loop controller according to claim 1 wherein a transfer functionof said model reference controller is K_(t)/(J_(m)s+B_(m)), whereinJ_(m) is a reference of a rotor inertia of a motor, B_(m) is a dampingcoefficient of a motor and K_(t) is a ratio.
 5. A current loopcontroller according to claim 4 wherein a rotor inertia control which ischanged according to a load of said servo system is controlled by saidmodel reference controller to be approximate to said reference of saidrotor inertia of said motor.
 6. A current loop controller according toclaim 4 wherein Jm and Bm of said transfer function,K_(t)/(J_(m)s+B_(m)), are according to a default of a specification. 7.A current loop controller according to claim 6 wherein saidspecification is a steady-state error of said servo system.
 8. A currentloop controller according to claim 1 wherein said current command signalis generated by said model reference controller according to adifference between said velocity command signal and said feedbackvelocity command signal.
 9. A current loop controller according to claim1 wherein said control signal is a voltage-controlled signal.
 10. Acurrent loop controller according to claim 1 wherein said control signalis a current-controlled signal.
 11. A method for controlling a currentloop in a servo system comprising steps of: generating a velocitycommand signal from a current command reference signal of said servosystem through a first operation; generating a current command signalthrough comparing said velocity command signal with a feedback velocitycommand signal of said servo system; and generating a control signalfrom said current command signal, said current command reference signaland a current feedback signal through a second operation in order todrive said servo system.
 12. A method according to claim 11 wherein saidservo system is an AC servo system.
 13. A method according to claim 11wherein said servo system is a permanent magnet servo system.
 14. Acurrent loop controller according to claim 11 wherein a transferfunction of said model reference controller is K_(t)/(J_(m)s+B_(m)),wherein J_(m) is a reference of a rotor inertia of a motor, B_(m) is adamping coefficient of a motor and K_(t) is a ratio.
 15. A current loopcontroller according to claim 14 wherein a rotor inertia control whichis changed according to a load of said servo system is controlled bysaid model reference controller to be approximate to said reference ofsaid rotor inertia of said motor.
 16. A current loop controlleraccording to claim 11 wherein said current command signal is generatedby said model reference controller according to a difference betweensaid velocity command signal and said feedback velocity command signal.17. A current loop controller according to claim 11 wherein said controlsignal is a voltage-controlled signal.
 18. A current loop controlleraccording to claim 11 wherein said control signal is acurrent-controlled signal.